A better way to solve it is let a and b be the number of units of coffee1 and coffee2, respectively. We take a weighted average:
For the LHS, the numerator represents the total cost of the coffee, the denominator represents the total # of units.
. That means, for every 1 unit of b (coffee2), we need 1.5 units of a (coffee1) to stay at $7.50 a unit. We like integer ratios so I'll multiply by 2, for every 3 units of coffee1 we need 2 units of coffee2.